I. Field of the Invention
This invention relates generally to printing or typing equipment involving the use of a direct thermal printing process or thermal transfer process to transfer a dry film impression onto an image carrying tape. More particularly, this invention pertains to an apparatus for controlling a thermal head associated with the printing machine.
II. Background of the Invention
In the field of commercial art, there is a significant need for a simple means for transferring prefabricated letters or characters to a "paste-up" sheet for later photographing and printing. The earliest technology involving letter transfers was that of dry rub-on transfer sheets which had a series of characters preprinted thereon. These materials, however, are limited by the number of characters available on the sheet and must be very carefully aligned to produce acceptable images. Machines were later developed which printed such letters on a continuous adhesive tape. The first such machines employed print disks having raised characters and used impact printing to transfer pigment to a carrier tape. Some machines were keyboard driven while others were manual "spin and print" machines. See, for example, U.S. Pat. Nos. 3,912,064, 4,243,333, 4,462,708 and 4,579,056.
The impact printing machines mentioned above had many advantages over the rub-on letters, but still suffered from certain limitations. Specifically, these machines were necessarily complicated, heavy, and relatively slow since the printing effect was accomplished by using physical force against the type face with the print media and pigment ribbon in between. Such machines were also restricted to pigment transfer of some form. In addition, an expensive type disk had to be molded for every point size, type style and language, leading to enormous costs in creating a suitable library. Certain foreign languages which are written either "backwards" or vertically would require very specialized type disks. Speed was further limited because the type disks had to physically move to a new location to print each successive character. Finally, the resulting output was generally not considered smear proof and would have to be further coated if used in areas where frequent contact with the print surface was anticipated.
Thermal transfer printing, such as that known in inexpensive portable typewriters (Canon Typestar 5.TM., etc.), employed a new technology which used a heat generating print head to melt a wax-like pigment from a carrier ribbon to a receiving tape. By using digital technology, characters could be formed of a series of pixels and no print disk was required. A related technology is direct thermal printing where an image is created on a thermally sensitive receiving paper directly by the head without the use of an intermediate carrier ribbon.
The key element in a direct thermal or thermal transfer system is the head, its alignment mechanism and the feeding device which precisely moves the receiving tape in synchronization with the activation of pixel elements on the head. In the above-mentioned typewriter devices, the pixel density is so low that only draft quality print is created and, thus misalignment of the head is not particularly serious. In the graphic arts, however, a much higher degree of accuracy for head alignment, paper feed and pixel density is required. This combination of circumstances creates special problems not previously encountered.
In thermal transfer, the pigment carrying tape is typically quite thin and fragile and, thus its surface must be well protected from mishandling by the user. Prior art devices such as the above-mentioned typewriter have employed cartridges, but they did not fully protect the ribbon from damage. The present invention solves this problem of damage to the tape and ribbon, as well as maintaining same in precise alignment.
An additional problem encountered in direct thermal and thermal transfer is the alignment of the head which is routinely exposed to the user. If the head is rigidly mounted so as to be accurately positioned, the mount may be damaged by the user. If the head is on a movable mount, it may not always return to its proper position. The present invention solves this problem by creating a mount which is both flexible, to resist damage, and designed to accurately return to its proper position. In addition to head alignment, the drive rollers which advance the tape must maintain accurate alignment despite the need for their retractability. The present invention likewise solves this problem.
In a printing apparatus, it is desirable to provide a mechanism to control the thermal head so that the head and its associated guide roller may be spaced apart to easily receive a tape and brought together in proper alignment when a tape is properly positioned between the roller and the head. Such an arrangement will facilitate installation of the tape. With respect to thermal heads, such control apparatus must include means to insure that the head is properly aligned when in its operable position to insure that any printed message is not distorted. Also, such apparatus should include means to permit the head to endure a user inflicted blow without misalignment of the head.